The present invention relates to the combustion of solid hydrocarbonaceous fuel such as coal, including fuel containing bound nitrogen, and to the reduction of generation of oxides of nitrogen in the course of that combustion.
Combustion of coal in the furnaces of power plants continues to be a significant means of generating energy. As that combustion continues to be believed to cause atmospheric emissions of NOx which continue to be considered to contribute to atmospheric pollution, there is still substantial interest in identifying ways to reduce the amount of NOx emitted to the atmosphere in the course of that combustion.
One mode of reducing NOx emissions is adopting staged combustion techniques either or both using aerodynamically staged low NOx burners and over fire air ports. In the aerodynamically staged low NOx burner, mixing with fuel of a portion of the combustion air required for complete combustion of the fuel is delayed to produce a flame with a relatively large fuel rich flame area within the flame. In globally staged combustion or staged combustion with over fire air combustion air, providing only a portion of the total amount of oxygen required for complete combustion of the coal, is fed to the burner in the primary combustion zone with the fuel to create a fuel rich flame area followed by a fuel lean area to which the balance of the combustion air (xe2x80x9cover fire airxe2x80x9d) is fed to complete the combustion of the fuel. The entire primary combustion zone, with the exception of the near burner area where combustion air is injected and not yet fully mixed with fuel, may become fuel rich under the globally staged combustion, providing a long residence time to reduce NOx emissions. To achieve fuel rich conditions, the prior art suggests reducing the amount of combustion air fed with the fuel to the primary combustion zone, adding combustible gases to this region, or using oxygen lean recycled flue gases. Adding pure oxygen at this point in the process is discouraged by prior teachings because it is inconsistent with the goal of eliminating excess oxygen from this area of the flame. In furnaces that burn pulverized coal, the pulverized coal is customarily conveyed to and fed through the burner as a flowing stream of the pulverized coal solids carried by and intimately mixed together with primary air (also know as xe2x80x9ctransport airxe2x80x9d). The transport air also provides a portion of the combustion air requirement for the coal. The transport air may contain recirculated flue gas or combustion products of fuel used by in-duct burners to reduce the moisture content of coal. Reducing the amount of transport air is one recognized technique for reducing the amount of NOx formed by combustion of the coal. However, in most situations, reducing the amount of transport air is not feasible as it adversely impacts the performance of the coal pulverizer and the transport/distribution system.
A second technique identified in the prior art for reducing NOx emissions from pulverized coal burners is the incorporation of coal concentrators or splitters. Coal concentrators or splitters inertially separate the coal from its transport air at or close to the tip of the burner into two or more separate streams of the pulverized coal and air, wherein one or more of the streams has a higher ratio of coal solids to transport air than one or more other streams. These devices inject the two (or more) streams into the furnace separately or highly stratified. In this fashion, the fuel rich stream burns under more fuel rich conditions, and the fuel lean stream would provide additional air and some heat to drive the devolatilization and denitrification of the coal. These devices also allow greater turndown of the burner, because as the coal loading decreases with decreasing firing rate (less coal and same amount of transport air), the concentrators allow denser coal streams at the burner tip which help maintain ignition stability of the flame. Techniques of this type have become quite common in the industry and there are now many coal concentrator devices designed for this purpose. However, the proposed NOx regulations from coal fired boilers have become very stringent and the use of the state-of-the-art low NOx combustion system with a coal concentrator alone is not sufficient to meet the new NOx regulations. Furthermore, the use of a coal concentrator can result in high unburned carbon in ash.
Thus, there remains a need for improved methods for carrying out combustion of coal so as to reduce the emissions of NOx, without significant increases in unburned carbon in ash, preferably while taking advantage of the features provided by coal concentrators.
The present invention comprises a method of combustion, comprising providing a feed stream of pulverulent solid hydrocarbonaceous fuel in a gaseous carrier,
obtaining from said feed stream at least one obtained stream comprising said fuel and said carrier and having a ratio of fuel to carrier that is higher than the ratio of fuel to carrier of said feed stream,
feeding said obtained stream and air from a burner to a combustion chamber, injecting oxygen into said obtained stream at or near said burner, such as by injecting it directly into said fuel as the fuel emerges from the burner or by adding oxygen to the air that is fed through said burner, and combusting the coal in said obtained stream in said combustion chamber with said air and oxygen in a flame that has a fuel-rich flame zone, wherein the amount of said oxygen is less than 25% of the stoichiometric amount required for complete combustion of said fuel and keeps the zone fuel-rich, while reducing the amount of air fed through said burner by an amount containing sufficient oxygen that the overall combustion zone stoichiometric ratio varies by not more than 10% compared to the stoichiometric ratio without said addition of oxygen.
In preferred embodiments, the fuel rich flame zone stoichiometric ratio is between 0.6 and 1.0 and more preferably between 0.7 and 0.85.
In another preferred embodiment, air is added from a source other than said burner into a region within said combustion chamber outside said fuel-rich flame zone to establish a fuel rich primary combustion zone, in an amount containing at least sufficient oxygen that the total amount of oxygen fed into said combustion chamber is at least the stoichiometric amount needed for complete combustion of said fuel. Preferably in this embodiment, the primary combustion zone stoichiometric ratio is between 0.6 and 1.0 and more preferably between 0.7 and 0.85.
It should be understood that the obtained one or more streams of coal and air having a higher ratio of fuel solids to air than that of the feed stream can be obtained, but are not necessarily obtained, as a stream or streams that are physically separate from the one or more streams of fuel solids and air of lower ratio of fuel solids to air that are also necessarily produced. That is, the obtained stream or streams of higher fuel solids to air ratio can constitute one or more regions that are part of a stream that also has one or more regions of lower fuel solids to air ratio.
The invention maintains the process goal of a high temperature, fuel rich flame zone by first concentrating the coal stream and then applying oxygen to the concentrated coal stream in a localized region at the exit of the burner. This allows a fairly high concentration of oxygen to be contacted with the coal and can maintain the stoichiometric ratio at or below the original air values depending on the degree of coal concentration achieved.
The combination of locally high concentrations of oxygen with low stoichiometric ratios creates ideal conditions for suppressing NOx formation. The use of oxygen in concentrated coal streams can achieve these conditions by excluding a portion of inerts, i.e., nitrogen contained in air, from the normal coal combustion process which allows higher temperatures to be achieved and results in overall lower NOx emissions from the process and less unburnt carbon in the ash. Also, by applying oxygen in this fashion, less oxygen is required to achieve the beneficial conditions desired, so the economics of oxygen usage is greatly enhanced.
As used herein, xe2x80x9cstoichiometric ratioxe2x80x9d means the ratio of oxygen fed, to the total amount of oxygen that would be necessary to convert fully all carbon, sulfur and hydrogen present in the substances comprising the feed to carbon dioxide, sulfur dioxide, and water.
As used herein, xe2x80x9cNOxxe2x80x9d means oxides of nitrogen such as but not limited to NO, NO2, NO3, N2O, N2O3, N2O4, N3O4, and mixtures thereof.
As used herein, xe2x80x9cstaged combustion with low NOx burnersxe2x80x9d means combustion in a furnace wherein mixing with fuel of a portion of the combustion air required for complete combustion of the fuel is delayed to produce a flame with a relatively large fuel rich flame zone
As used herein, xe2x80x9cglobally staged combustionxe2x80x9d or xe2x80x9cstaged combustion with over fire airxe2x80x9d means combustion in a furnace wherein only a portion of the combustion air required for complete combustion of the fuel is fed to the furnace with the fuel at the burners, and additional air (the xe2x80x9cover fire airxe2x80x9d) constituting at least enough air to complete the combustion of the fuel is fed to the furnace not through or immediately adjacent any burner but instead through one or more inlets situated between the burner(s) and the furnace flue means, and is fed without an associated feed of fuel.
As used herein, xe2x80x9cgaseous carrierxe2x80x9d means a gaseous medium with no oxygen or with an oxygen content suitable to transport the fuel without a risk of ignition occuring in the fuel system, with the balance of the gaseous medium constituting inert species or the products of combustion from a prior process. In practice, the volume flow of the gaseous carrier must be adequate to entrain and displace the full size range of pulverized fuel particles.
As used herein, xe2x80x9cbound nitrogenxe2x80x9d means nitrogen that is part of a molecule that also contains carbon and hydrogen and optionally also oxygen.